IP Multimedia (IPMM) is an example of a service that provides a dynamic combination of voice, video, messaging, data, etc., within the same session. By increasing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, e.g. peer-to-peer multimedia communication, IPTV etc.
These services can be based on the IP Multimedia Subsystem (IMS) architecture, which is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks (3GPP TS 22.228, TS 23.228, TS 24.229, TS 29.228, TS 29.229, TS 29.328 and TS 29.329 Releases 5 to 7).
The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Other multimedia applications which can be used for media transmission and control include Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP), Message Session Relay Protocol (MSRP), and Hyper Text Transfer Protocol (HTTP).
FIG. 1 illustrates schematically how the IMS fits into the mobile network architecture in the case of a 3GPP PS access network.
Call/Session Control Functions (CSCFs) operate as SIP proxies with the IMS. The 3GPP architecture defines three types of CSCFs: the Proxy CSCF (P-CSCF) which is the first point of contact within the IMS for a SIP terminal; the Serving CSCF (S-CSCF) which provides services to the user that the user is subscribed to; and the Interrogating CSCF (I-CSCF) whose role is to identify the correct S-CSCF and to forward to that S-CSCF a request received from a SIP terminal via a P-CSCF.
When a user wishes to access services provisioned by an IP Multimedia Subsystem (IMS) network, the user must typically register with his or her home network (this may occur at power-on of the user's device). An IMS/SIP client in the user equipment initiates the registration process by sending a SIP REGISTER message to the Serving Call Session Control Function (S-CSCF) allocated to the user within the home network. On receipt of the REGISTER message, the home network can allow or deny access to the IMS network by the user. A user's device or User Equipment (UE) communicates with the IMS via an access network, and the first point of contact for the user within the IMS is the P-CSCF. As part of the registration process the S-CSCF stores the contact address of the P-CSCF together with the address of the UE.
The signalling between the terminal and the P-CSCF is carried out using the access network. It is possible that the link for signalling information in the access network is lost. This may come about, for example, when a “through-air” signal is lost (for example when a user passes through a tunnel) or when a user moves into a congested cell. There is ongoing work in 3GPP R7 to ensure that the P-CSCF will receive an indication that the signalling link has been lost, for example via the Rx interface. However, no proposals have been made as to what the P-CSCF will do with this indication, except a simple P-CSCF generated de-registration. In other words, the P-CSCF would send a SIP REGISTER request to the S-CSCF for deregistration of the terminal. This is not generic or flexible enough to indicate the terminal's signalling connectivity status. The IMS currently contains no mechanism for transporting a loss of signalling link indication.